The present invention relates to an energization control apparatus for a glow plug in a diesel engine.
A glow plug extends into each combustion chamber of a cylinder head to facilitate smooth starting of a conventional diesel engine in a cold season. At the start of an engine operation, the glow plug is energized and heated to increase a temperature of compressed air in the cylinder head, thereby assuring starting of the diesel engine. In order to improve operability of the diesel engine to a degree equivalent to a gasoline engine, demand has arisen for a glow plug which has preheating time of almost zero second before the start of the engine. In general, in such a glow plug, a power is controlled through an energization control apparatus which is operated upon connection of a key switch to an ON mode position. A high power is supplied to the glow plug to achieve rapid heating. For a certain period of time after rapid heating, a low power is supplied to the glow plug to achieve stable heating. In general, stable heating of the glow plug upon starting of the engine is called an after glow operation. The interior of the combustion chamber can be warmed up by the after glow operation, and at the same time, knocking of the diesel engine can be prevented. In addition, generation of noise and white smoke and exhaust of the HC component can be prevented.
The power supply cycle of the glow plug by using the above energization control apparatus is generally performed as follows. In the initial energization period, a battery voltage (normally 8 to 10 V) is directly applied to the glow plug to obtain the above high power for rapid heating. When the temperature of the glow plug reaches a predetermined temperature by rapid heating, the glow plug is connected in series with a voltage-drop resistor (i.e., a dropping resistor) to lower the voltage applied to the glow plug, thereby obtaining the low power for the after glow operation. FIG. 19 is a graph showing energization control characteristics when the low power for the after glow operation is obtained by using the dropping resistor. At time P1 after a lapse of two seconds upon the start of energization, the voltage applied to the glow plug can be decreased by connecting the dropping resistor thereto.
In addition to the direct voltage drop by means of the dropping resistor, a low power for the after glow operation can also be obtained by the following indirect method. The voltage for rapid heating is intermittently applied to the glow plug, and the intermittent time is duty-controlled to decrease an RMS voltage. This method is described in Japanese Patent Laid-Open (Kokai) No. 59-122782. An energization control apparatus for the glow plug in this prior art produces a low power supplied to the glow plug during stable heating by intermittently applying a power source voltage to the glow plug. That is, a voltage of an envelope obtained by connecting ON peaks of the voltages intermittently applied to the glow plug is generated to obtain a reference voltage which is continuously changed such that a magnitude of the reference voltage is small for a high envelope voltage and large for a low envelope voltage. The average voltage of the ON and OFF times of the voltages intermittently applied to the glow plug is always compared with the reference voltage. Negative feedback control is performed to cause the average voltage to coincide with the reference voltage or set the average voltage in proportion thereto. An RMS voltage applied to the glow plug is kept constant regardless of the ON voltage applied to the glow plug. That is, the low power supplied to the glow plug is kept constant regardless of variations in power source voltage. Therefore, the temperature of the glow plug during stable heating can be kept constant.
As indicated by the energization control characteristics in FIG. 20, an ON/OFF time of the voltage applied to the glow plug is duty-controlled at time P2 after the start of energization of the glow plug. The RMS value of the voltage applied to the glow plug is decreased to obtain the low power for stable heating. The duty ratio is set to be variable according to the voltage applied to the glow plug, as shown in FIG. 21. Therefore, the low power supplied to the glow plug during stable heating is kept constant regardless of changes in values of the voltages applied to the glow plug.
In the conventional energization control apparatus for the glow plug described above, when a key switch is connected to an ON mode position and at the same time a starter mode position to achieve cranking during the severe wintertime (strict winter condition) which corresponds to an outer air temperature of -10.degree. C. or less, (i.e., when quick starting of almost zero second is to be performed), an excessive cranking power is required, and a battery voltage is greatly lowered (up to 6 to 7 V in a normal operation condition). As a result, a voltage applied to the glow plug to achieve rapid heating thereof cannot be sufficiently assured. That is, since the glow plug cannot be rapidly heated at the time of starting of the engine, its starting characteristics are greatly degraded, and quick starting of almost zero second is difficult to perform.
The conventional energization control apparatus for the glow plug has an advantage in that the low power supplied to the glow plug during stable heating can be kept constant. However, this apparatus cannot compensate for a decrease in glow plug temperature against fuel spray swirl generated in the combustion chamber during cranking of the engine. That is, negative feedback control of the low power supplied to the glow plug can be performed to compensate for a decrease in power source voltage during cranking of the engine. A control value is given as a predetermined value. When the fuel spray swirl generated inside the combustion chamber acts on the glow plug during cranking, a decrease in temperature of the glow plug cooled by this fuel spray swirl cannot be compensated, thereby degrading the starting characteristics of the engine.
In addition, since the low power supplied to the glow plug during stable heating is kept unchanged and when an ambient temperature is decreased, a glow plug temperature is decreased accordingly. In severe wintertime (severe winter condition) having an ambient temperature of lower than a predetermined temperature (e.g., -15.degree. C.), starting characteristics of the engine are degraded.
According to the conventional method of controlling energization of the glow plug as described above, in an extremely low temperature state (i.e., severe winter condition) having an outer air temperature of -15.degree. C. or less, even if the key switch is connected to the ON mode position and at the same time a high power is supplied to the glow plug to achieve rapid heating, the starting characteristics of the engine are not necessarily improved. That is, a viscosity of fuel (light oil) in the severe winter condition is increased, and a spray particle size is increased. When the key switch is connected to the ON mode position and at the same time the high power is supplied to the glow plug, the temperature of the glow plug is excessively increased at the start of fuel injection during cranking, thus resulting in poor ignition.
After the start of cranking, a perfect combustion time, i.e., "rev-up" time from the first ignition to perfect ignition is prolonged. In particular, in a severe winter condition, the perfect combustion time is further prolonged.